In many clinical situations, it is desirable for physicians to monitor a patient's cardiac output. Typically, a patient's cardiac output is expressed in terms of the volume (in liters) of blood ejected by the left ventricle of the patient's heart within a one minute period. In a mathematical sense, cardiac output (CO) may be defined as the product of heart rate (HR) and stroke volume (SV), as follows:HR (heartbeats per minute)×SV (liters per heartbeat)=CO (liters per minute)
Various techniques have been used for measurement of cardiac output, including direct measurement methods as well as indirect measurement methods. Typical methods for direct measurement of cardiac output include thermodilution and indicator dye dilution. Typical methods for indirect measurement of cardiac output include thoracic bioimpedance, Doppler ultrasound and a technique known as the Fick method whereby cardiac output is calculated using a formula that is based on the measured oxygen contents of samples of mixed venous blood (i.e., blood obtained from the patient's pulmonary artery) and arterial blood (i.e., blood obtained from one of the patient's arteries) as well as the measured carbon dioxide (CO2) content in air expired from the patient's lungs.
The thermodilution method for measuring cardiac output is frequently used in clinical practice. Typically, thermodilution cardiac output measurements are carried out through the use of a special type of catheter known as a Swan Gantz right heart catheter, sometimes referred to as a pulmonary artery or “PA” catheter or thermodilution catheter. The Swan Gantz right heart catheter is a balloon-tipped catheter that is inserted into a vein (typically the internal jugular vein, external jugular vein, subclavian vein or brachial vein) and initially advanced to a first position where the catheter's distal tip is positioned in the right atrium of the patient's heart. While the catheter is in this first position the balloon located near the catheter's distal tip is inflated. The flowing blood then carries the inflated balloon (and the distal end of the catheter) through the right atrium, through the tricuspid valve, through the right ventricle, through the pulmonic valve and finally to a second position wherein the distal tip of the catheter is situated in the patient's pulmonary artery. This procedure is sometimes referred to as “flow directed” catheter placement. After the distal end of the catheter has been flow directed into the pulmonary artery, the balloon is deflated. Thereafter, as the catheter remains indwelling, the balloon may occasionally re-inflated for brief periods of time to facilitate measurement of a variable known as “pulmonary artery wedge pressure.” This is accomplished by causing the balloon to substantially block flow through the pulmonary artery and then obtaining a pressure reading within the pulmonary artery, distal to the inflated balloon.
A thermistor is located near the distal end of the catheter. The thermistor is connected to a cardiac output computer. The thermistor may be a fully sleeved, single beaded thermistor made of nickel alloy insulated with polyimide (Sensors of these types are available from, for example, Biosensors International Pte Ltd 1995). Bead thermistors of this type can provide good stability and, in at least some cases, provide accuracy similar to that of more expensive platinum resistance thermometers (PRTs). Bead thermistors are also relatively fast and able to monitor temperature changes over a very short period of time, a characteristic that may be useful in the thermodilution method of determining CO.
A proximal injectate port is formed in the portion of the catheter that resides in the right atrium or vena cava when the catheter's distal tip is in the pulmonary artery. When it is desired to measure cardiac output, a bolus (e.g., 10 cc) of room temperature or cooled injectate (e.g., saline solution or 5% dextrose in water) is injected through the proximal injectate port, becomes mixed with the flowing blood and is carried through the right ventricle and through the pulmonary artery. As the cooled blood/injectate mixture passes the thermistor, the thermistor's resistance changes. The amount of change in resistance is proportional to the change in blood temperature. A voltage across the thermistor generates a small current. That current changes as the temperature sensed by the thermistor changes and, thus, generates a signal representing the temperature sensed by the thermistor. The cardiac output computer receives the temperature signal from the thermistor and, on the basis of the change in temperature monitored by the thermistor following injection of the injectate, calculates cardiac output (liters per minute). For example, the injection of a bolus of cold fluid may mix with the blood and create a bolus of cold blood which will cause a transient rise in temperature as the blood passes the thermistor in the PA, and the time in which the temperature sensor detects a rise in temperature will determine how fast that bolus of blood is passing by the thermistor, i.e. velocity of the blood flow, from which the CO can be determined.
In addition to facilitating measurement of cardiac output by thermodilution, the typical Swan Gantz catheters (i.e., pulmonary artery catheters) have also incorporated multiple working lumens that terminate in ports located at various location on the catheter body. These lumens and their accompanying ports are useable for infusion of fluids, withdrawal of blood samples and for monitoring of blood pressures and pressure wave forms at various locations in the right heart. For example, samples may be taken of venous blood from a proximal port located in the vena cava or right atrium, or mixed venous blood samples from a distal port located in the pulmonary artery. Also, for example, central venous pressure (CVP) may be monitored through the proximal port located in the vena cava or right atrium, right ventricular pressure (RVP) may be monitored through a medial port located in the right ventricle, pulmonary artery pressure (PAP) may be monitored (while the balloon is deflated) through the distal port located in the pulmonary artery and pulmonary artery wedge pressure (PAP-W) may be monitored (while the balloon is inflated) through the distal port located in the pulmonary artery.
The traditional Swan Gantz or PA catheters have required that a bolus of injectate (e.g., cool 0.9% NaCl solution) be injected into and mixed with the patient's blood each time it is desired to obtain a reading of cardiac output. Such bolus injections of saline solution or other injectate can be problematic. For example, if the patient is hypothermic, it can be necessary to ice or refrigerate the injectate prior to its introduction into the body in order to ensure that the injectate temperature is sufficiently different from the blood temperature to provide a meaningful cardiac output computation. Also, in cases where the proximal injectate port of the Swan Ganz catheter remains inside of the introducer sheath some portion of the injectate bolus may flow in the retrograde direction within the sheath, thereby resulting in delivery of less than the full bolus volume to the pulmonary artery and a resultant error in the cardiac output determined. Also, in patients who are hypertensive or very ill, the volume and chemical composition of the injectate can cause undesired effects on the patient's electrolyte balance, state of hydration, blood pressure, etc.
Recognizing the potential problems associated with repeated bolus injections of saline solution or other injectate for the purpose of thermodilution cardiac output measurements, others have described “injectateless” thermodilution catheters wherein heat is exchanged between the catheter and the flowing blood in a manner that can allegedly be detected by a thermistor located in the pulmonary artery and from which the patient's cardiac output can be computed, but which does not require the introduction of any foreign substance into the blood. Examples of such “injectateless” thermodilution catheters include those described in U.S. Pat. No. 4,941,475 (Williams et al.) entitled Thermodilution By Heat Exchange and U.S. Pat. No. 5,807,269 (Quinn et al) entitled Thermodilution Catheter Having A Safe, Flexible Heating Element. Although these prior art “injectateless” thermodilution catheters may be useable for measuring or estimating cardiac output and for other purposes typically required of catheters of this type, these prior art devices are not believed to be optimal for use in all patients and/or for all clinical purposes. Thermodilution cardiac output catheters are typically not designed for, or capable of, substantially changing the body temperature of the patient, as such is not their intended purpose. There are also significant risks and negative health consequences associated with floating a catheter through the heart and into the PA. Accordingly, there remains a need in the art for the development of new “injectateless” catheters and related devices/methods for measuring cardiac output.